1. Field of the Invention
The present invention relates to a nitrogen-containing sintered alloy comprising a hard phase, and more particularly, it relates to a nitrogen-containing sintered alloy which is improved in thermal shock resistance, wear resistance and strength for serving as a material for a cutting tool and enabling application to wet cutting.
2. Description of the Background Art
A nitrogen-containing sintered alloy having a hard phase of a carbo-nitride mainly composed of Ti, which is bonded by a metal binder phase containing Ni and Co, has already been put into practice as a cutting tool. This nitrogen-containing sintered alloy is widely used for a cutting tool in a manner similar to the so-called cemented carbide, which is mainly composed of WC, since the hard phase is much finer than that of a conventional sintered alloy that is free of nitrogen, whereby the high-temperature creep resistance is remarkably improved.
In this nitrogen-containing sintered alloy comprising a hard phase, however, the resistance against thermal shock is reduced for the following reasons. First, the thermal conductivity of this nitrogen-containing sintered alloy comprising a hard phase is about half that of the cemented carbide, since the thermal conductivity of Ti, which is the main component of the carbonitride, is extremely smaller than that of WC, which is the main component of the cemented carbide. Secondly, the thermal expansion coefficient of the nitrogen-containing sintered alloy comprising a hard phase is about 1.3 times that of the cemented carbide, since this coefficient also depends on the characteristic value of the main component in a manner similar to the thermal conductivity.
Therefore, the nitrogen-containing sintered alloy has a disadvantageously inferior reliability as compared to a coated cemented carbide or the like in cutting operations that cause a particularly strong thermal shock such as milling, cutting of a square timber with a lathe, or wet copying with remarkable variation in depth of cut, for example.
In order to solve such problems of the conventional nitrogen-containing sintered alloy comprising a hard phase, various improvements have been attempted as follows. For example, Japanese Patent Laying-Open No. 2-15139 (1990) proposes means of improving surface roughness of a material containing at least 50 percent by weight of Ti in terms of a carbide or the like and less than 40 percent by weight of an element belonging to the group 6A (the group VIB in the CAS version) of the periodic table in terms of a carbide and having an atomic ratio N/(C+N) of 0.4 to 0.6 with a high nitrogen content by controlling the sintering atmosphere, for forming a modified part having high toughness and hardness in a surface layer thereof. On the other hand, Japanese Patent Laying-Open No. 5-9646 (1993) discloses a cermet prepared by sintering a material mainly composed of Ti and containing less than 40 percent by weight of W, Mo and Cr in total in terms of a carbide, and thereafter controlling a cooling step for providing a surface layer of the cermet with a region having a smaller amount of binder phase as compared with the interior, to leave compressive stress on the surface.
However, each of the cermets disclosed in the aforementioned Japanese Patent Publications is insufficient in chipping resistance as compared with the coated cemented carbide, although wear resistance and toughness are improved. Further, the cermet is so inferior in thermal shock resistance that sudden chipping is easily caused by thermal cracking or crack extension resulting from both thermal and mechanical shocks in particular, and sufficient reliability cannot be attained. Although the manufacturing cost for such prior art materials is reduced due to the omission of a coating step, the performance cannot be sufficiently improved. Thus, the prior art suggests that the potential improvement in strength against chipping is naturally limited in the category of the so-called cermet, which is prepared on the premise that it contains Ti in excess of a certain amount.
The inventors have studied and analyzed cutting phenomena such as temperature distributions in various cutting operations and arrangements of material components in tools, and have thereby recognized the following.
During cutting, a cutting member is partially exposed to a high-temperature environment, for example at a surface of an insert that is in contact with a workpiece, a part of a rake face that is fretted by chips, and the like. The thermal conductivity of the cermet is about half that of the cemented carbide as hereinabove described. Hence, heat that is generated on the surface of the cermet is so little diffused into the interior that the temperature is abruptly reduced in the interior although the surface is at a high temperature. Once cracking is caused in such a state, the cermet is extremely easily chipped. Furthermore, when the cermet is rapidly quenched with water-soluble cutting oil from a high temperature state or cooled with cutting in lost motion, only an extremely small part of its surface is quenched.
Furthermore, the thermal expansion coefficient of the cermet is about 1.3 times that of the cemented carbide as hereinabove described. Hence, tensile stress is caused on a surface layer, which extremely easily causes thermal cracking. In relation to either characteristic, the cermet is inferior in thermal shock resistance to the cemented carbide.
Comparing the cermet and the cemented carbide having the same grain sizes and the same amounts of binder phases, the fracture toughness of the former is reduced by about 30 to 50% as compared with the latter. Hence, crack extension resistance is also reduced in the interior of the alloy.
In the conventional nitrogen-containing sintered alloy with a hard phase, as hereinabove described, there are limits to the improvement of thermal conductivity, reduction of the thermal expansion coefficient and improvement of crack extension resistance that can be achieved with a large content of Ti, and that can achieve an excellent machined surface in a manner that is advantageous in view of the resource.